This application relates to an apparatus for facilitating measuring borehole data and for transmitting the data to the surface for inspection and analysis. Although the subject invention may find substantial utility at any stage in the life of a borehole, a primary application is in providing real time transmission of large quantities of data simultaneously while drilling. This concept is frequently referred to in the art as downhole measuring while drilling or simply measurements-while-drilling (MWD).
The incentives for receiving reliable downhole measurements during drilling operations are substantial. Downhole measurements while drilling will allow safer, more efficient, and more economic drilling of both exploration and production wells.
Continuous monitoring of downhole conditions will allow immediate response to potential well control problems. This will allow better mud programs and more accurate selection of casing seats, possibly eliminating the need for an intermediate casing string, or a liner. It also will eliminate costly drilling interruptions while circulating to look for hydrocarbon shows at drilling breaks, or while logs are run to try to predict abnormal pressure zones.
Drilling will be faster and cheaper as a result of real time measurement of parameters such as bit weight, torque, wear and bearing condition. The faster penetration rate, better trip planning, reduced equipment failures, delays for directional surveys, and elimination of a need to interrupt drilling for abnormal pressure detection, could lead to a 5 to 15% improvement in overall drilling rate.
In addition, downhole measurements while drilling may reduce costs for consumables, such as drilling fluids and bits, and may even help avoid setting pipe too early. If in the event MWD allowed elimination of a single string of casing, further savings could be achieved since smaller holes could be drilled to reach the objective horizon. Since the time for drilling a well could be substantially reduced, more wells per year could be drilled with available rigs. The savings described would be free capital for further exploration and development of energy resources.
Still further knowledge of subsurface formations will be improved. Downhole measurements while drilling will allow more accurate selection of zones for coring, and pertinent information on formations will be obtained while the formation is freshly penetrated and least affected by mud filtrate. Furthermore, decisions regarding completing and testing a well can be made sooner and more competently.
There are two principal functions to be performed by a continuous MWD system: (1) downhole measurements, and (2) data transmission.
The subject invention pertains to the data transmission aspect of MWD. In the past, several systems have been at least theorized to provide transmission of downhole data. These prior systems may be descriptively characterized as: (1) mud pressure pulse, (2) insulated conductor, (3) acoustic and (4) electromagnetic waves.
In a mud pressure pulse system the resistance to the flow of mud through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar sub near the bit.
The communication speed is fast since the pressure pulse travels up the mud column at or near the velocity of sound in the mud, or about 4,000 to 5,000 fps. However, the rate of transmission of measurements is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of transmitting data in a fairly noisy environment.
Insulated conductors, or hard wire connection from the bit to the surface, is an alternative method for establishing downhole communications. The advantages of wire or cable systems are: (1) capability of a high data rate; (2) power can be sent down hole; and (3) two way communication is possible. This type of system, however, has at least two disadvantages; it requires a wireline installed in or attached to the drill pipe and it requires changes in usual ring operating equipment and procedures.
One hard wire method is to run an electrical connector and cable to mate with sensors in a drill collar sub. The trade off or disadvantage of this arrangement is the need to withdraw the cable, then replace it each time a joint of drill pipe is added to the drill string. In this and similar systems the insulated conductor is prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string. Also, cable techniques usually entail awkward handling problems, especially during adding or removing joints of drill pipe.
As previously indicated, transmission of acoustic or seismic signals through a drill pipe, mud column, or the earth offers another possibility for communication. In such systems an acoustic (or seismic) generator would be located near the bit. Power for this generator would have to be supplied downhole. The very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission. Moreover, signal-to-noise limitations for each acoustic transmission path are not well defined.
The last major previously known technique comprises the transmission of electromagnetic waves through a drill pipe and the earth. In this connection electromagnetic pulses carrying downhole data are input to a toroid positioned adjacent a drill bit. A primary winding, carrying the data for transmission, is wrapped around the toroid and a secondary is formed by the drill pipe. A receiver is connected to the ground at the surface and the electromagnetic data is picked up and recorded at the surface.
In conventional drillstring toroid designs a problem is encountered in that an outer sheath which must protect the toroid windings must also provide structural integrity for the toroid. Since the toroid is located in the drill collar, large mechanical stresses will be imposed on it. These stresses include tension, compression, torsion and column bend. This structural problem is quite significant when it is realized that: (1) in prior toroid designs the conductive drill collar was attached at both ends to the outer sheath of the toroid, (2) such structure provided a path for a short circuited turn and (3) in order to prevent short circuits a peripheral insulation gap in the drill collar was required notwithstanding the severe environmental loading.
The problems and unachieved desires set forth in the foregoing are not intended to be exhaustive but rather are representative of the severe difficulties in the art of transmitting borehole data. Other problems may also exist but those presented above should be sufficient to demonstrate that room for significant improvement remains in the art of telemetering MWD borehole data.